1. Field of the Invention
The present invention relates generally to a glass antenna device for vehicles of the type wherein two antennas are provided on the same surface of a fixed window glass, such as a rear window glass or a side window glass, for the reception of FM and TV broadcasts and AM broadcasts, respectively, and a transformer is connected to transmission lines extending from the two antennas for performing the impedance conversion of the transmission lines. More particularly, it relates to a vehicle window glass antenna device designed to prevent reductions in the AM broadcasts reception sensitivity.
2. Description of the Related Art
Conventional vehicle window glass antenna devices are equipped with a choke coil to prevent a reduction in the reception sensitivity. The present assignee has proposed in Japanese Patent Laid-open Publication No. HEI 9-018222 a glass antenna device equipped with a transformer devoid of choke coil, such as shown here in FIG. 4 of the accompanying drawings.
As shown in FIG. 4, the proposed glass antenna device 51 includes an exclusive antenna 53, a compatible antenna 52 formed by defogging heater conductors 55 connected to bus bars 54 (54a, 54b), and an impedance conversion transformer 57. A primary winding of the transformer 57 is connected at its one end 57a to the exclusive antenna 53 and at its middle point 57b to the compatible antenna 52 via a lead. A secondary winding of the transformer 57 has terminals 57c, 57d connected to a center conductor 56a and an outer conductor 56b of a coaxial feeder cable 56. By virtue of the impedance conversion achieved by the impedance conversion transformer 57, a practically sufficient degree of reception sensitivity can be obtained even though a conventional choke coil is eliminated.
Because of the impedance conversion transformer associated with the exclusive antenna to eliminate a choke coil, the glass antenna device disclosed in Japanese Patent Laid-open Publication No. HEI 9-018222 achieves practically sufficient reception sensitivity.
However, there is room for improvement in that signal attenuation may occur when a cable interconnecting a receiver-side impedance conversion transformer and an AM receiver is long and hence has a large distribution capacitance.